Win XP, Off XP, SBS 2000

I have recently upgraded 17 PCs as above with a new SBS 2000 network. We are experiencing difficulty sharing files (which are saved in share on the server) "Sometimes" SBS appears not to recognise when a user saves and closes a file - and access to others is later denied. However, it doesn't always say "who" was the last user. The only way the file can be closed is either 1) closing the file manually on the server or 2) logging the user out of the network.
Any ideas - my engineers have other clients with same configuration and same problem - any ideas please!